Method of production of anhydrous fluorophosphoric acid



Patented July 15, 1947 UNITED STATES FATE v Tf f QFFlCE METHOD OF PRODUCTION OF ANHYDROUS 'FLUOROPHOSPHORIC ACID Willy Lange, Cincinnati, Ohio, and Ralph Livingston, Chicago, Ill., assignors, by direct and mesne assignments, to Ozark-Mahoning Com pany, a corporation of Delaware No Drawing. Application September 26, 1944,- Serial No. 555,883

Claims. 1

Our invention relates to the production of anhydrous monofluorophosphoric acid H2PO3F and anhydrous difluorophosphoric acid I-IPO2F2 since in accordance with the specific manner in which it is practised either or both of them can be prepared thereby.

In United States Letters Patent 2,408,784, granted October 8, 1946, on an application filed by us jointly, the anhydrous monofluorophosphoric acid and a method of producing it from anhydrous hydrofluoric acid (HF) and water-free metaphosphoric acid (HPOa) are disclosed and claimed, and in United States Letters Patent 2,408,785, granted October 8, 1946, on an application filed by Willy Lange, a method of producing the monofiuorophosphoric acid alone or in conjunction with the difluorophosphoric acid from phosphorus pentoxide and hydrofluoric acid is disclosed and claimed. However, until theinvention of the present method there was no way of producing anhydrous difluorophosphoric acid alone on a commercial scale so far as we are aware and it therefore differs from the aforesaid methods not only in this particular, 1. e., in its ability to produce the difluorophosphoric acid, alone, but basically in the chemistry by which it,

is produced.

Chemical literature discloses that the ions of difluorophosphoric acid are formed by hydrolysis of phosphorus oxytrifluoride (POF3) according to the equation POF3+H2O=HPO2F2+HF, but the difluorophosphoric acid prepared in this manner cannot be obtained in an anhydrous, pure state as the hydrogen fluoride formed enters-into secondary reactions producing impurities which,

cannot be easily removed. Moreover a further obstacle in the hydrolysis of phosphorus oxytrifluoride is the fact that hydrolysis of the P OF3 is not completed before hydrolysis of the difluorophosphoric acid begins with resultant formation of 'monofluorophosphoric acid and hydrogen fluoride. Our investigations have demonstrated that these reactions are not simple ones but that equilibria are formed some of which are expressed by the following equations:

and HzPOzF+H2OHsPO4+HFL It results that hydrolysis of phosphorus oxytrifluoride with Water does not afford a pure anhydrous monofiuorophosphoric or difluorophosphoric acid in economical yields.

, Reactions similar to those just indicated also occur when phosphorus pentoxide is heated'with calcium fluoride at high temperatures and the 2 phosphorus oxytrifluoride, formed in this reaction,'reacts with moisture present in the reaction chamber. However, the quantities of difluorophosphoric acid obtainable in this manner are'so small that the process is of nocommercial interest as a method of producing it.

We'have discovered that the fluorine atoms in phosphorus oxytrifluoride and difluorophosphoric acid may under proper conditions be easily exchanged with hydroxyl groups of orthophosphoric and monofluorophosphoric acids, that is, in the series PO(O*I-I) 3, POF( OH) 2,v POF2 (OH) and POFs,

in which :n is ascribed any value between and 2 inclusive; in accordance therewith if a: moles POF3 are reacted with 1 mole HaPOi approximately the following molar yields of 'monofluorophosphoric and'difluorophosphoric acids are obtained:

i i I Theoreticalyields in Moles Value of x in terms of Moles M0nofluom Difluom; I phosphoric phosphoric Acid Acid The values of 1c in this-table are given by way of: example. only as it will be understood that a: may beascribed any value between 0 and 2 with correspondingly modified results. However, if a: is less than A, a corresponding quantity of monofiuorophosphoricacid is formed but in mixture with the surplus of water-freeorthophosphoric a'cid,'and as this mixture cannot be separated into is .expressedby the following a ileae's phoric acid only is obtainedt It;w-be.-aseribeda;

higher value than 2, with resultant surpluseof P0193, no additional reaction is produced.

In accordance with the reaction?-typified' by this equation an almost pureanhydrous difluoro phosphoric acid may be obt'airied"byreacting? phosphorus oxytrifluoride upon water-free ortno product is =of =a grade suffici-entlyapure, for com! mercial punposes However-itmay be i further-" purified :by -.dis tillationunder reduced-pressure it ms-- s Likewise --a...,mix ture of rno nofluorophosphoric.

and difluoroph o sphoricacidsmay belobtained by- :PQQ D h ytriflueride-upon.waterfre'e orthophosphoric v acid --inwhich: case, equation- 1- becomes 2" phosphoric! acidrsbeingsobtainedi in: substantially? pure iformzas.tthesliisti-llatexwhile'thmmonofluoroe phosphoric acid-remains-as the distillation-residue andrri's-rxof: a technical' grade; apparentiy; not; susceptible toe-further pur'iflcaitionby-waeuumidise" tillationzoraotherwisee Further, anhydrous monofluorophosphoricsacidF alone- :may: her-obtained:- by:'reactingephosphorus oxytrifiuoride uponvwatersfree orthophosphorim acid in which case Equation 1 becomes:

i. e., PO(OH)3+ /2POF3=1 /2POF(OH)2. Equa-. tions 2 and 4 thus represent special cases of the" general Equation 1 in which:tli1 -variable -":z isa accorded respectively-fits m'aXiir'i-tiimand minimum values of 2. andJ/z while. in Equation 3 it isaccorded an intermediate fvaluiof ll.

The reactions 'do not take. place" iristan; ta'ne'ously, but-require. time to go dicorripletionf increase in temperature 'enhaneiri'gfjthjrat of reaction as 'will'b' readily understood." Priorlto't the stage of the flnal products being reache'din' these reactions, however, intermediate stages are passedas'canb demonstrated by known analytical methods and of which brief mention may be made;- Thusi dur-irigq the reaction between waterfreecrthophosphoric acid and phosphorus oxytrifluoride-inthe-ratio fi-l mole-HsPOl-r l mole-POP? (Equation-3); at some-intermediate stage-more difiuorophosphoric acid'is' 'present than"mono fluorophosphoricacid and accordingly some still unreae-tedorthophosphorieacidislikewisepresent .wherieby-itheziorthoplrosphoric acid -rea;cts'" with the :difluorophosphoricaiaci'd 'toi-formzmondfluorophosphoric sacidiaecording-aoitlie equation otherii's the reaction of the monofiiioropho'sphoric 4 tion of difluorophosphoric acid according to the equation:

i. e5,.POF(OH)2+POF#POF OH). We have also-discovered that theSereactions designated as (5) and (6) may be carried out in an isolated system, using pure components, outside the general-reactionw-mixture to which we have referred and 'cor isequntlyin this way either the monofliiorophosphoric or the difluorophosphoric acid mayflomroduced.

v 'Ifhevariousreactions typified by Equations 1-6 take place quantitatively and at any temperature from"comp'aratively low to comparatively high 011857-7'01518 .rate'of' reaction depending upon the differ'nce in the number of fluorine atoms present in theitwo types of molecules reacting with each otherand' upon the reaction temperature. In none of them is water or hydrogen fluoride formed so that the above mentioned-equilibria do not exis' t 'in th practi-ceof ourinvent-ion which" constitutes'one of stsampmmm features.

we are fully' aw are ofthe possibility that interr-n'ediary' reactions other than those "to which we have referred may take place in therea'ction' products" obtai "ed--- by introducing phosphorus oxyti i fluori'de intoorthophosphoric acid or mono fluoropl-iosphoric' aeid; but knowledgefthereof; if in faotthey d'o' oecur, is unnecessary forthe pra'oti'c'a'P employment- 0f our inventiomsirice the Te? actionsj-typifiedby Equations-154 inclusive though complex intiiemselves finally go to completion With-bi Without-the applicat-idn "of heat, -and"the" intermediary reaction Equations 5 and" 6 also go tocon'ipletion- '-when'- carried out an isolated System5 It follo'wsthat in "such employment;

whateverbathe-specific procedureadopted, satis factory results maybeiobtained' as "long'at"waterfr'ee orthophosphoric acid is reacted with phos'- phorus =oxytrifluoridejin the ratio 'of' 1 mole HaPbicrc moles "POFs in' wh'ich r-is aseribediany value between and? in hisivesor anhydrous -monofluorophosphoric" aoid is realcte'd -with phosphorusoxytrifiiiorid*in" a ratio of 1 mole rnrb'rrfz'i man e-1: 51.

with anhydrous? difiiioropliosp heric aci'd-- in amums-:1 mme mnom meirerrPot-m ln the' praotic'e 'o the inverition heat is ap view to -increas iiig th rarividitif of closed yesseFto-suppress th tendency of 'difliidr'o' peratures' higher than about c." and-tr i cneniiempnos pnbnc id tofstartfdecomposfgi tiomas wmperatures *hi'ghefith-an ab'out"160" ct;

tr'o'ducing therea'ction components into" acl'osed" rotary retort in the form of a slightly inclined gear:driverrfcylifidenwhicE is"heated heatingifchamber through? which ZmbVBS" a corrstant stream 'of 'a mixtur'e of the reaction" conttainer.

ponents, the gaseous phosphorus oxytrifluoride being constantly developed in the cylinder and led to the reaction vessel containingorthophosphoric or monofiuorophosphoric acid and the solid reactionp-roduct being continuously withdrawn at the lower end of the reaction" cylinder with the help of a special sea-ling device which prevents loss of POFs and entrance of, air; the reaction may also be carried out by using fiat boxes, open atthe top and partly filled with the reaction components, whichare passed in an endless procession through a tunnel, heated from the outsideand'closed at both ends with special sealing device-s.v v

Moreover, instead of using phosphorus oxytrifluoride prepared in the foregoing or any other convenient way, we may employ the gaseous mixture' formed as the Icy-product of the reaction of fiuoroapatite with small quantities of phosphorus pentoxide and consisting of phosphorus oxytrifluoride, silicon tetrafluoride and carbon dioxide.

The use of thismixture in the practice of our method results in afairly slow reaction because of the dilution ofthe reactive component, namely POFs, with inert matter; it is therefore desirable to condense 'the'mixture and fractionally distill it in order to obtain a more concentrated phosphorus oxytrifluoride. This product is eminently suitable for the purpose.

' The phosphorus oxytrifiuoride may be reacted with the orthophosphoric or monofluoropho'sphoric acid by introducing the gas in a fine dispersion near the bottom of a reaction vessel containingthe acid, the vessel being desirably elongated vertically so that the gas passes through the acid. for a relatively great distance, thus affording opportunity for a large part of the phosphorus .oxytrifluoride .to be absorbedby the acid. Especially near the end of the reaction, however ,'some of the gasescapes from the acid unabsorbed and this escaping gas is preferably removed from near the top of the closed reaction vessel and pumped back or otherwise returned to its gas inlet. Another way of effecting the desired reaction is to stir the acid in an atmosphere of POF3 in a closed vessel equipped with a fast running stirrer, thus offering large constantly formed fresh surfaces to the gas Which is replaced from a storage container as it is absorbed, while still another is to spray the acid in a fine dust into a reaction chamber filled with phosphorus oxytrifluoride which is constantly replenished from a suitable source, or in lieu of any of these ways of bringing about the reaction any other manner of suitably producing it may be employed. Where the reaction is to be efiected between orthophosphoric acid and difiuorophosphoric acid alone in accordance with Equation 5, it may be best carried out by merely mixing the components in an appropriate con- The following are illustrative of various ways of practising the method of our invention:

Example 1.4.9 parts by weight of liquid waterfree orthophosphoric acid were placed in a vertically elongated glass reaction chamber having near its bottom a perforated disk forming the end of a gas inlet tube to thereby effect a fine distribution of the gas bubbles emanating from the disk, the chamber being closed and provided near its upper end with an oulet for nonabsorbed gas. The chamber was arranged for disconnection from the means for supplying the POFs so that by weighing it the quantity of the latter absorbed by the orthophosphoric acid could be determined. The entire equipment was made of Pyrex glass.

Phosphorus oxytrifluoride was passed through the acid until 9.6 partsby weight were absorbed and the reaction product then consistedof difiuoroe phosphoric acid which, as hitherto indicated, was capable of purification by distillation under reduced pressure, the acid boiling at about 70 C. at a pressure of 200 mm. of mercury. The reaction in this'example was therefore in accordance with Equation 2.

Example 2.4.9 parts by, Weight of liquid waterfree orthophosphoric acid were placed in the re,- action chamber and reacted with phosphorus oxytrifluoride as described in Example 1. After 5.7 parts by weight of the acid had been absorbed, introduction of POF3 was discontinued and the reaction product kept in the closed container at C. for 4 hours. The reaction product then, as ascertained 'by suitable analytical methods, consisted of about equal parts by weight of monofluorophosphoric and difiuorophosphoric acids, the latter being capable of separation in vapor form from the product by distillation under reduced pressure, the monofluorophosphoric acid remaining as the distillation residue. I' his example was therefore in accordance with Equation 3.

Example 3.--4.'9 parts by weight of liquid water-free orthophosphoric acid were reacted with 2.8 partsby weight of phosphorus oxytrifluoride in the manner in Example 1 and using the same equipment, the reaction product being kept for 9 hours at 80 C. in the closed reaction vessel. Analysis showed the product to be free of unreacted orthophosphoric acid and of noticeable quantities of difluorophosphoric acid and to consist substantially of anhydrous monofluorophosphoric acid only. This example was there- .fore in accordance with Equation 1.

hydrous difluorophosphoric acid and covered with a silver lid; after the formation of heat had ceased and the crucible and contents had been maintained for 8 hours at 80 C., the difluorophosphoric acid had disappeared and the reaction products consisted of monofluorophosphoric acid only. This example was therefore in accordance with Equation 5.

All the foregoing examples numberd 1-5 inclusive are given by way of illustration only and not in any restrictive or limiting sense since many other ways may be employed for combining and subsequently treating the initially supplied components in order to conform to special requirements of the equipment being utilized and other determining factors.

The reaction vessels may be those customarily utilized for similar or analogous operations and may be made of stainless steel, silver or other corrosion resistant material or may be plated therewith and, according to conditions of the reaction, ordinary or pressure vessels may be used.

It will now be apparent that by the practice of the invention either anhydrous monofluorophosphoric. acid or anhydrous difluo'rophosphoric acid or: both:- may be; readily producedzloy-zexchange of fluorine. atoms withhydroxyl groups "through re action betweenranhydrous compounds of-the gem eral'stype POFn(OH)3 n in .whichn represents .0, .1,' 2-,'-orsz3ehy.miinng=two compoundsof this general typeashowing a difierence in the number ofthe fluorine atoms: in their molecules ,Of,2- or;3. Thus our invention for the first time asgfarras werareaware, makes possible the production. of anhydrous difluorophosphoric, .acid alone; while it also :maybe practisedto produce: that acid in; conjunction with anhydrousmonofluorophosphoricsacidxor the latter acid alone, all of said acids being. in a: form suitablejfor, industrial; ,uses andtheimethod itself being capable of convendent, economical operation in apommerc-ial W y- Havingtthus described; our invention; we claim and desire to protect Joy 1 Letters Patent of the Unitedgfitates +2(a:-/z)I-IPO2F2 inwhichxr is ascribedany-value between ,/2 and 2 inclusive.

3. The method ofproducing anhydrous difluorophosphoric acid which comprises mixing anhydrous monofluorophosphoric acid-withphosphorus oxytrifiuoride. v

4'. The meth'odto'f producing a mixture ofanhydrous monofluorophosphoric and difluorophosphoric acids which comprisesmixing phosphorus 6.- The method of producing anhydrous monofluorophosphoric and difluorophosphorio acids which comprises mixing 5 phosphorus oxytrifiuoride with water-free orthophospho'ric acid in the ratio of 1 mole H3PO'4: 1 mole POF; according to the equation H3PO4+POF3=H2POaF+HPO2Fa 7 The. method of producing anhydrous difiuorophosphoric acid which comprises mixingv phosphorus oxytrifiuoride 'withwater-free orthophosphoric acid in the ratio of 1 mole H3PQ42. 2 moles POE: according to the equation v 8. ,The method of produing anhydrousdifiuorophosphoric .acid which comprises mixing phosphorusoxytrifluoride with anhydrous monofiu'orophosphorio acid in the ratio 0131 mole, I-IzPOriF: 1 mole PO'F3 accordingto the equation" HzPO3F+POF3=-'2HPO2F2 9. The method of producing anhydrous monoii'uoropllosphoric acid which comprises mixing water-free orthophosphoric a ci'd'and anhydrous difluorophosphoricacid in the ratio of 1 mole HPO i2 1 mole I-IPOzFa according to the equation HsPO4+ HPQ2F2=2H2PO3F 10. The-method of-producing a mixture-oranhydrous monofiuorophosphoric and difiuorophosphoric acids which comprises mixing waterfree o'rthophosphoric acidwith phosphorusoxyt'rifiuoride in the ratio-of 1 mole HzPOi to more than mole and less than 2 moles POFs;

-W '1LLY'LANGE; RALPH LIVINGSTON; 

